Studies of cell kinetics in patients with acute leukemia are being done. In order to determine the proliferative activity of the cell population, blood and bone marrow samples are obtained. The mitotic indices are measured and the number of cells in DNA synthesis determined autoradiographically from the labeling index with tritiated thymidine. Currently studies are being done to determine the effect of chemotherapeutic agents singly and in combination on the proliferative activity of the leukemic cells. These results will be used in the design of chemotherapeutic regimens for acute leukemia. Studies are also being done in patients with abnormal but non- malignant myelopoiesis in order to learn more concerning variations of normal proliferation and maturation patterns. In vitro and in vivo studies will be designed in order to obtain information concerning the growth regulatory mechanisms for leukemic and non-leukemic marrow cells. Infections are a major problem for leukemic patients. Therefore studies of cellular and humoral defense mechanisms in these and other patients having an apparent increased predisposition to infection will be done.